1. Field of the Invention
The present invention relates to a scheme for determining a three-dimensional position of a radio transmitter. More specifically, the present invention relates to a scheme for accurately estimating a three-dimensional position of an RFID (Radio Frequency Identification) tag by using RFID tag system.
2. Description of the Related Art
In the RFID tag system, an RFID tag with an unique identification (ID) recorded therein is attached to an object. The object is identified by ID information (tag ID) included in the RFID tag and is received by an RFID antenna.
Some types of RFID tag receives electric power from radio waves received from the RFID antenna. This type of RFID needs not individual power sources. Thus, the device may be downsized by the size of the power source. This type of the RFID tag system may be applied to a small and lightweight object to be detected. That widens the usage of that kind of RFID tag system.
The RFID tag system has a problem of low accuracy in estimating a position, when a tag is placed relatively far away because of the characteristics of the system. A conventional method for estimating a position of the RFID tag is to provide a plurality of RFID antennae in a workspace to roughly detect the presence of the RFID tag near the RFID antenna that receives radio waves from the RFID tag.
In the method for estimating a position of the RFID tag proposed Intel Corporation, a two-dimensional position of the RFID tag is determined using probability scheme such as the Bayes' rule as can be seen in; “Mapping and Localization with RFID Technology”, Matthal Philipose et al., Proceedings of the 2004 IEEE International Conference on Robotic and Automation. In the method, an RFID antenna carried by a robot detects an RFID tag at a measurement point in a workspace. Probability (likelihood) of presence of an RFID tag near the measurement point is determined. The process is repeated as the robot moves in the workspace to obtain a distribution of the likelihood values in the workspace. The measurement point with the highest likelihood determined to be the point at which an RFID tag is present.
When roughly detecting a position of an RFID tag from a position coordinate of an RFID antenna that receives radio wave transmitted from the RFID tag as in the conventional method, however, the larger the communication distance is, the bigger is the region where the RFID tag that has transmitted the radio wave may be present. Accordingly, it is difficult to determine the position as a coordinate.
The method of Philipose et. al. performs position estimation on a two-dimensional plane only. If the method is expanded to three-dimensional position estimation, it requires a great number of samples as well as time in position estimation of the RFID tag.
The present invention provides a method for estimating a three-dimensional position of a radio transmitter (RFID tag) more accurately and quicker than the conventional techniques.